This invention relates to a safety device for a tilting bed of an X-ray diagnostic apparatus etc., which prevents a rotation of a bed beyond a limit rotation angle.
A tilting bed for an X-ray diagnostic apparatus etc., is so constructed that it is tiltable to permit a human subject (patient) to be held at a desired diagnostic angle anywhere from a horizontal position to a standing position. A safety device is provided on the tilting bed to prevent a rotation of the latter beyond a predetermined tilting angle. FIGS. 6A and 6B show one example of a conventional tilting bed device for X-ray diagnostic apparatuses which is equipped with the aforementioned safety device. FIGS. 7A and 7B are an enlarged view showing portions A and B in FIG. 6A, i.e., major parts of the aforementioned safety device.
That is, tilting bed device 1 shown in FIGS. 6A and 6B includes bed section 2, support base 3 for supporting the bed section, sector gear 4 of a semicircular configuration, and rotation shaft 5 fixed to the sector gear. One end of shaft 5 is rotatably supported on support base 3. To the other end of shaft 5 is rotatably attached a pinion gear 9 which is in mesh with a guide rail rack for the aforementioned bed section 2. Gear grooves 4a are provided on the outer periphery of the sector gear 4 and are in mesh with drive gear 8 provided on the inside surface of the support base 3.
A chord section defined at the upper edge of sector gear 4 faces upwardly on a top plate side of bed section 2 and a pair of cam followers 6, 6 are located inwardly in the neighborhood of the chord section of sector gear 4 in a spaced-apart relation and are projected in a direction of bed section 2. Guide rail 7 is mounted on the bed section side and has rail groove 7a of a U-shaped configuration formed, on the top plate side, along the longitudinal direction of the bed section with cam follower 6 kept in engagement with rail groove 7a. Pinion gear 9 on rotation shaft 5 engages with rack 7b located on the lower surface of guide rail 7. A drive motor, not shown, is connected to drive gear 8. When drive gear 8 is driven in a direction of, for example, arrow a.sub.1 in FIG. 6A, the sector gear in mesh with drive gear 8 is rotated in a direction of arrow a.sub.2 with rotation shaft 5 as a center to permit bed section 2 to be tilted in accordance with its rotation angle. Pinion gear 9 is coupled through a transmission, not shown, to the motor for drive gear 8. The rotation in a direction of arrow b.sub.1 of pinion gear 9 causes guide rail 7 to be moved, in a direction of arrow b.sub.2, through a rack 7.sub.b which engages with pinion gear 9. Thus, bed section 2 can be tilted at a desired rotation angle anywhere from the horizontal position to the standing position of the patient. Furthermore, since the bed section is raised in the tilting direction during the rotation of the pinion gear, there is no possibility that the bed section will come into contact with a floor.
The raising of bed section 2 in the tilting bed is achieved by the longitudinal movement of guide rail 7 which engages with cam follower 6. Where an excess rise of the bed section, such as a state of the disengagement of the bed section from guide rail 7, takes place, the bed section is removed away from the support base, causing an accident to occur on the patient. As a countermeasure against a possible accident, a safe provision is made for the conventional device with restriction placed on the angle through which the bed section is tilted, thereby preventing rotation of a bed beyond that tilting angle. In this connection, a first safe measure is adopted which comprises setting a limit rotation angle beyond a normal tilting range relative to the rotation of the sector gear, locating a limit switch actuating piece in proper place relative to the surface position of the sector gear corresponding to the limit rotation angle, and actuating the limit switch, upon the rotation of gear 4 to an extent corresponding to the limit rotation angle, interrupting a power supply to the motor and thus preventing further rotation of the sector gear. Second and third safe measures as set out below can also be adopted according to this invention. That is, a stopper is provided as a mechanical means as shown, for example, in FIGS. 7A and 7B. In the arrangement shown in FIG. 7A, a metal piece 4b is deposited by, for example, welding onto the groove of the last gear tooth of a gear train of the sector gear, that is, at a location past the limit rotation angle position where the aforementioned limit switch actuating piece is placed. In the arrangement shown in FIG. 7B a blocklike stopper 7c is fitted into the rail groove of guide rail 7, preventing the detachment of the cam follower from the rail groove.
The aforementioned stopper prevents sector gear 4 and guide rail 7 from being rotated and moved, respectively, beyond their limit range. Since an inertial force acts upon the bed section on which the weight of the patient has been heavily inflicted, there is a greater risk that the stopper will be destroyed due to the collision of drive gear 8 or cam follower 6 with the stopper. The stopper acts as such when bed section 2 is rotated beyond the limit rotation angle, that is, when the limit switch as a control means for an off-normal rotation of the bed section fails. In other words, the stopper serves as such when the bed section is operated without noticing any off-normal state in which the off-normal rotation control mean has been placed. The repetitive collisions of the drive gear and cam follower with the stopper due to the rotation of the bed section beyond the limit rotation angle cause a mechanical fatigue to be gradually induced in the stopper, resulting in a eventual breakage of the stopper. Since the stopper is provided as a final safety securing means for the tilting bed device, the tilting bed has a greater risk of being injured or destroyed, giving a fatal below to the bed section. Thus, there is a growing demand for a safer tilting bed device.